Removed price placeholders

These placeholders look too much like real prices, confusing users.

ECOM-2075

ecommerce/static/templates/professional_course_seat_form_field.html

@@ -11,7 +11,7 @@
             <div class="input-group">
                 <div class="input-group-addon">$</div>
                 <input type="number" class="form-control" name="price" id="price" min="5" step="1" pattern="\d+"
-                       placeholder="100" value="<%= price %>">
+                       value="<%= price %>">
             </div>
         </div>
     </div>

ecommerce/static/templates/verified_course_seat_form_field.html

@@ -12,7 +12,7 @@
             <div class="input-group">
                 <div class="input-group-addon">$</div>
                 <input type="number" class="form-control" name="price" id="price" min="5" step="1" pattern="\d+"
-                       placeholder="100" value="<%= price %>">
+                       value="<%= price %>">
             </div>
         </div>
     </div>